Start by preheating your oven to 400°F. This ensures your scallops cook evenly and get that golden, buttery finish.
Place scallops in a sauté pan with white wine and a pinch of salt. Bring to a boil, then reduce to a simmer, cover, and cook for about 4 minutes. This gentle cooking keeps them tender.
Melt 3 tablespoons butter in the same pan and sauté onions until soft, about 2 minutes. The onions give the sauce a natural sweetness that complements the scallops perfectly.
Sprinkle flour over the butter and onions, cooking briefly. Whisk in the reserved cooking liquid to create a smooth, creamy sauce. This sauce is what makes these scallops irresistible.
Arrange the scallops in the casserole dish and pour the sauce over them. Sprinkle breadcrumbs on top and bake for 15–20 minutes until golden and bubbly.